Implement new thumbnail loading strategy

- By default, we load only the reduced resolution screenshots.
- As soon as the user stops scrolling fast, we also start loading
full resolution screenshots.
- We prefetch reduced resolution screenshots when scrolling from
back to front, as the other direction is automatically prefetched
because the thumbnails aren't immediately visible.

Test: Open many apps, adb restart, scroll fast and slow in recents
Test: runtest systemui -c com.android.systemui.recents.model.HighResThumbnailLoaderTest
Bug: 34829962
Change-Id: I7f7a9842eb28a09a18573426fa9677cee2877124